Diversity combiner control system



Aug. 2, 1960 M. s. ULSTAD ET AI. 2,947,861

DIVERSITY COMBINER CONTROL SYSTEM Filed Sept. 29, 1958 2 Sheets-Sheet 1 l o /l/ l `I NcsE-cu-IoFF VARIABLE IIE ICIFICUIT I .3

20 RECEIVER'I I o THRESHOI-.D CONTROI- SIsNI. GENERA-lan #I 7 Y V i l WEIGHTIN( S IGNAL I I ow .Ill

THR ESHOl-.D CONTROL. SIGNAL.

MEREDITH 5. ULsrAD HowARo D. HERN ATTORNEYS United iStates Patent l2,947,861 DIVERSITY COMBINER CONTROL SYSTEM Meredith S. Ulstad and Howard D. Hem, Cedar Rapids, Iowa, assignors to Collins Radio Company, Cedar Rapids, Iowa, a corporation of Iowa Filed sept. 29, 1 958, ser. No. `763,928 s Claims. (C1. 25o-20) This invention relates to combiner circuits for diversityoperated angular-modulation receivers. The invention is particularly applicable to diversity arrangements of variable-threshold angular-modulated receivers of the type described in patent application No. 696,473 by Howard D. I-Iern, filed November 14, 1957, titled Method and Means for Reducing the Threshold of Angular-Modulation Receivers, and assigned to the same assignee, as the present application.

Prior diversity combiners are known which simultaneously combine outputs of F.M. receivers on a weighted basis according to ratios of detected noise obtained from respective noise channels, that select noise-energy in noise-frequency bands above the signal-frequency band.

In order to provide a noise-energy band above the signal' band at the output of aA discriminator, it is neccessary `in prior systems to provide an LF. bandwidth sufficiently wide to accommodate the required noise for proper diversity combining.

In a variable-threshold receiver of the type in the above-cited application No. 696,473, theIF. bandwidth available near minimum threshold cannot accommodate sufficient noise-energy components outsideV the signal-frequency band for proper diversity combination. In variable-threshold receivers, a resulting fluctuation in I.F. bandwidth under low signal conditions annihilates the proportionality between the detected noise and the required combining function. The invention does not use noise-energy components for obtaining weighting signals used in controlling a diversity combiner.

It is therefore an object of this invention to provide a signal combiner which will operate with variable-threshold angular-modulation receivers.

It is a further object of this invention to provide a diversity combiner that is compatible with F.M. receivers with fixed LF. bandwidths,

It is another object of this invention to provide a diversity combiner which does not require a second power ing circuit. Y

It is still another object iof this invention to vprovide a diversity combiner which prevents noise feed-through by 'any given receiver when it is below threshold, while another receiver Vin the Ydiversity arrangement iskabove threshold.

The invention is used with a plurality of receivers which are arranged foreither space or frequency diversity reception.' Each of the receivers includes means for heterodyning a received'signal to an ILF. frequency. A logarithmically-limiting I F; amplifier respectively receives the output of each receiver. A weighting-signal generator is connected to stages Yof the I.F. amplifier to logarithmicallydetectthe received signal and providethe weighting signal.v` I discriminators are connected respectively'to the outputs of the hunting LF. amplifiers to detectithereceived Vsignal simultaneously. f

source for the returni current in a cathode-follower add- '-,Aunique load'is provided Ywith acathode-follower type of combiuer'to permit the elimination of a negative power supply for the'ca'thode-follower current return.'

The load consists of a fixed-biased transistor, which proy vides a low resistance'to direct-current,` but provides very high impedance to signal-frequency alternating. currents enabling a maximum .amount of degeneration that is needed for optimum combiner action.` 1 The weighting signaly is provided to the cathode follower to control its bias.

When any F.M. receiver is below threshold, its limiter is captured by noise, which becomes the strong output of its discriminator. criminator prevents feed-through of noise to thecombiner when the respective-receiver drops below threshold.v

The noise cutoff circuit includesy a phase-splitter which divides a respective discriminator output into two parts of opposite phase. One part is connected to the grid of the respective cathode-follower, and the other part is connected to the cathode Vof the same Vcathode-follower. Cancellation of the discrirninator output occurs in the cathode-follower only when its grid is biased below cutoff by disconnecting the weighting signal.

The weighting-signaldisconnect circuit is provided toV drive the respective cathode-follower far below cutoff when a respective receiver is below threshold. The disconnection is required because the weighting signal does not appreciably drop-olf below threshold due to captured noise. The weighting-signal disconnect circuit is also operated by a control signalderived from the logarithmic LF. amplifier. The threshold-control-signal in variablethreshold receivers may be used, since it must change appreciably when the signal approaches threshold. The weighting-signal disconnect circuit comprises a directcurrent amplier that'receives, amplies, and may change the potential level and polarity of the threshold-control signal before applying it as a bias to the control grid of a respective cathode follower.

Further objects, features, and advantages of the invention will be apparent to a person skilled in the art upon further study of the specification and'the accompanying drawings, in which:

Figure 1 illustrates a formv of the invention;

Figure 2 shows a schematic of one' type of circuitry which may be used in somev of the blocks illustrated in Figure l; and A Figures 3 and 4 show curves used in explaining the operation of the invention.

'I'he drawings are now considered for an of an embodiment of the invention.'

In Figure l a dual-diversity system is illustrated. It will become obvious after the specification is studied that the invention can be used with diversity systems having any number of receivers.V The two receivers in the system of' Figure 1 are identical and therefore it is only necessary to describe one in` detail. system comprisingitems10 through 39 'is identical to the receiver system comprising items 50 through S9.

v The diversity system in Figure '1 is presumed to be a spiace-diversity system, wherein'antennas 10 and-50 are explanation spaced from each other so that multipath fading by the signal receivedby one of them does not have a high to antenna 10 and heterodynes the received signalV to an LF. frequency level, which is provided atV its'output.

A multi-stage LF. amplifier and limiter 13 receives theY output ofreceiver 11. Limiter 13 comprises eight stages of serially-connected Iamplitude-limiting circuits," each 4stage having a unique limiting range.'V The last stage #8 limits on the lowest antenna signal level and the A noise-cutoff circuit with each dis- Thus, the receiver v f- V2,947,861 A, K

3 other stages limit at consecutively higher antenna signal levels, until stage #1 limits only the highest level signals. Each of the limiter stages has the same gain,

prior to limiting by it; and they peak limit within successiveV ranges of receivedsignal level.

A frequency-modulation discriminator 25 receives the amplitude-limited output of amplifier 13 "and detects the signal carried by the received wave.

A noise-cutoff circuit 18. receives the detected signal from discr-iminator 25. Circuit 18 includes tube 19 connected asV a phase-splitting circuit. It has resistors 21 and 22 respectively connected as plate and cathode load resistors. 'The signal is therefore provided in oppositephased parts from the plate and cathode of tube 19. The in-phase part is coupled through a blocking capacitor 24 tothe grid of cathode-follower tube 28. The phaseinverted part is coupled through a capacitor 23 to the cathode of tube 28. The-level of the cathode signal applied through capacitor 23 is made equal to the level of signal coupled from grid-to-cathode through the interelectrode capacitance of the tube. Thus, the value of capacitor 23 and/or resistor 21 is chosen to control the proper signal level to the cathode. The noise-cutoff functioning of this circuit is explained below.

The cathode outputs of tubes 28 and 78 of circuits 18 and 68"are combined across a signal-adding circuit 91. Circuit 91 comprises an NPN ytransistor 96, which has its emitter connected to ground through a resistor 94 and its collector connected to the cathodes of both tubes 28 and 78. A resistorV 92 connects a B-plus voltageV source to the base of transistor 96; and a Zener diode 93 is connected between the base and ground. Diode 93 maintains a fixed bias between ground and a base of transistor 96.

The added output is provided at la terminal 120 and is taken from the collector of transistor 96. Figure 4 illustrates the relationship between the direct-current flow and the alternating-current impedance provided by transistor 96. This load characteristic provided by transistor 96 obtains a uniquely efficient cathode-follower function. The low direct-current resistance absorbs only a small amount of power from the direct-current source and eliminates the need for a negative direct-current return source required with previous resistor loads. Furthermore, thevery high alternating-current impedance provide'd by the transistor maintains aV maximum amount of degeneration for each cathode-follower which is necessary for obtaining optimum signal combining.

Where the current load is large, two or more transistors, each being connected as transistor 96, can be connected in parallel. The emitter resistor 9,4 assists in maintaining the desired alternating-current impedance characteristic of transistor 96.

Antenna 50 receives the `same transmitted signal as antenna 10, except that the fading Icharacteristic of the received signal is diiferent atl the two antennas. Consequently, the same signal is detected by discriminators 25 and 75 of the two diversity receivers, except that the signal-to-noise ratios at the discriminator output vary in a random manner with fading. A weighting-signal output from a generator 14 will Yvary according to the received signal-to-noise ratio at a respective receiver, as long as the received signal is above minimum threshold. The weighting signal is a D.C. type of signal applied as a bias to fthe grid of a respective cathode follower to control its impedance to the detected signal. The greater the signal -received by `an antenna, the greater is the weighting signal provided from the respective generator 14, and the higher above cutoi is the respective cathodefollower tube driven. Vice versa, when the weighting signal of a respectivereceiver drops in value its cathode follower presents a correspondingly greater impedance to the signal on a logarithmic basis, as is determined by the logarithmic characteristic of generator 14.

Weighting-signal generator 14 isv connected t@ ICSPQC' response.

4 tive stages of amplifier-limiter 13. The respective generator stages detect the outputs of the respective limiter stages yand serially add them together on a direct-current basis. This provides a direct-current output from generator 14 which is logarithmically related =to the abovethreshold signal level received at LF. input 12.

A low-pass filter 17 isconnected to the output of generator 14 to attenuate noiseand alternating-current components higher in frequency than the uctuating directcurrent signals, which should only be a function of the fading characteristic at antenna 10. The weighting signal is provided through low-pass iilter 17 and an isolation resistor 26 to the grid of cathode follower 28 to control its bias.

Because of the diversity-receiver arrangement in Figure l, it is most likely that when one receiver falls below its minimum threshold, the other receiver will be above minimum threshold to provide a useable signal. However, with the receiver below threshold, its limiter 13 captures noise rather than signal and provides a strong noise voltage output, which is undesirable but unavoidable under these circumstances. Under such circumstances it is essential that the captured noise of the belowthreshold receiver not be transmitted to signal-adding circuit 91, since it would deteriorate the quality of the output signal. The weighting-control signal from filter 17 does not decrease sufliciently to drive its cathode-follower tube very much below cutolf. In Figure 3, line 131 represents a typical above threshold weighting signal, which is semilogarithmically linear (the abscissa only in Figure 3 is represented on a logarithmic basis) above theV minimum threshold valve 136. Below the minimum input-threshold-signal level 136, the output voltage from weighting-signal generator 14 varies according to dashed line 132. Thus, the output voltage from generator 14 does not drop appreciably when the threshold recedes below minimum due to `the noise components being captured in the limiter. Consequently, the change in weighting signal at minimum threshold is insuflicient to properly cut off tube 28 to prevent any substantial feedthrough of noise.

' In order to provide a sharp cutoff bias for tube 28, disconnect circuit 30 is provided7 which is controlled by threshold-control-signal generator 16. Disconnect circuit 30 acts, in eiect, to ground the output of low-pass filter 17 when the input signal is below minimum threshold.

Threshold-control-signal generator 16 is provided with two stages, which are respectively connected to outputs of the fifth and sixth stages in amplifier-limiter 13. The stages of threshold generator 16 may be designed in the same manner as the stages of weighting generator 14, and they also will provide an approximate logarithmic Only two stages are used in threshold generator 16 because of 'the type of use for its output. A basic use for the output of generator 16 is to control the -variable bandwidth of Vthreshold receiver 11, as described in above-cited patent application No. 696,473. The choice of limiter stage #5 as a rst input connection to generator l16 is determined for each receiver because peak limiting ceases in limiter stage #5 when the receiver input signal drops below its maximum threshold. Curve in Figure 3 illustrates a typical response for the output voltage of threshold control generator 16.

During the substantially flat portion of curve 130, a receiver 11 is maintained at its maximum bandwidth to minimize intermodulation distortion. When the signal amplitude recedes below maximum threshold, the directcurrent signal recedes below the knee of curve 130. As the signal further recedes in amplitude, the bandwidth of receiver 11 further decreases and the RM. threshold of the system dropscorrespondingly.V At some value of the threshold-control-signal, minimum bandwidth and threshold arereached within the receiver 11. Minimumbandwidth point 141 iS illustrated in Figure 3 at vertical range of threshold is provided between the minimum and maximum values. When the signal falls below minimum threshold level 136, the threshold control voltage varies according to the dashed line extension of curve 130.

The maximum threshold is determined by the maximum LF. bandwidth within receiver 11. Thus, as long as the receiver is above maximum threshold, limiters and #6 are peak limiting; and an output 26 of generator '16 is at a maximum direct-currentlevel which maintains maximum bandwidth in receiver 11. However, when the input signal drops until the threshold recedes below maximum, the signal level within limiter #5 drops below clipping level; and the output voltage from generator 16 becomes lower to decrease the bandwidth within receiver 11 and to lower its F.M. threshold. When the signal drops still further in amplitude, the next limiter stage #6 eventually drops below its peak-clipping level; and the output of generator 16 continues to decrease the receiver bandwidth and the EM. threshold.

The output voltage of generator 16 drops at a steep rate with the input signal level below maximum threshold, and the generator 16 output voltage provides a positive indication when the minimum FM. threshold of the receiver system is reached.

The output of generator 16 operates weighting-signal disconnect circuit 36, which includes a tube 32 that has its control grid connected through an isolation resistor.v

31 to generator output '20. Disconnect circuit 30 is basically a direct-current amplifier and potential-level changer. The D.C. amplified output of tube 32 is provided across a resistor 36, but the direct-current level of the output is dropped to a lower level by means of a Zener diode 34 connected between resistor 36 and the plate of tube 32. A transistor 38, which is NPN, has its base connected through a resistor 37 to a point between diode 34 and resistor 36. Transistor 38 has its emitter connected to ground through a resistor 39 and has its collector connected through a resistor 26 to the grid of a cathode-follower tube 28. The valueof resistor 39 is chosen to provide a bias on the transistor to operate it in a common base manner rather than in a common-emitter manner, because of the higher A.C. irnpedance obtainable with the former manner. Asthe output of threshold generator 16 drops,` the output of transistor 38 drops atV asteep'rate. The DC. Alevel is adjusted with respect tothe 'normally/expected cutoff level of tube 28 so that when the generator 16 output indicates the minimum threshold is reached, tube 28 is disconnected by being biased far` below cutoff yby the voltage output' of thedisconnect circuit. At some D.C.v bias, such as level 134 inl-ligure 3, tube 28 is driven' below cutoff; and when the signal falls substantially below threshold 136, the bias is far below level 134 and tube 28 is far below cutoff. Thus, the threshold-control voltage, shown by curve 130 in Figure 3, switches transistor 38 to a very low collector-to-ground impedance, when it reaches the level represented by point 141 in Figure 3. Then, the grid of cathode-follower 28 is effectively grounded; and due to the high D.C. cathode voltage on tube 2S, it is held far below cutoff. The drop in value of the weighting-voltage below threshold due to the operation of disconnect circuit 36 is represented by dotted line 133 in Figure 3.

The cutoff is also a function of the signal level at the output of the other receiver system, due to the parallel combination of the receivers with tubes 2S and 78 across circuit 91.

Neverthless, even if a cathode-follower tube is below cutoff, there will be some noise passed to the output of the diversity system through the grid-to-cathode interelectrode capacitance of the tube. Noise-cutoi-circuit 18 prevents such extraneous feed-through. When tube 28 the threshold level of the F.M. system. In practice, as much as a ten decibell is below cutoff, the electron current in thetube stopped;

and the interelectrode coupling provides the only feed through from the grid. Consequently, this feed-through and the opposite-phased equal-level signal applied through capacitor 23 cancel in signal-adding circuit 91 and can not cause any noise output.

FigureV 2 illustrates a schematic diagram of some of the multiple stages used in I.F. amplifier-limiter 13 and generators 14 and 16, which have been found particularly suitable in practice. Since the stages of amplifierlimiter 13 may be identical, only Vone stage need be explained in detail. Thus in Figure 2, stage #5 comprises a pair of triodes 501 and 502 which act as amplifiers when the input signal is .below a certain level and act as peak clippers when the input signal exceeds a given higher level. vThis type of amplifier-limiter stage is well known. Tubes 501 and 502. Vare cathode-coupled. Tuber 501 is a cathode-follower, and tube 502 is a grid grounded amplifier which has its output provided to the next stage through a blocking capacitor 505 connected to its plate. lTuned circuit 503 is connected as the plate load of tube S02 and is a broadband LF. filter. A cathode load is comprised of an inductance 506 and a resistor 507, that are connected between the common cathodes and ground. The cathode circuit has a 10W impedance which is particularlysuitable for coupling to diode-detector types of circuits, which have low impedances. The other stages in amplifer-lirniter 13 can be made in the same manner as described stage #5.

Stage #5 of weighting-signal generator 14 is cornprised of a diode 511 which has its opposite ends connected through a pair of blocking capacitors 508 and v 509 to opposite ends of the cathode circuit in limiter stage #5. A load resistor 514 in series with a choke coil 512 is'connected across diode 511, and a capacitor 513 -is connected across resistor 514. The direct-current component detected by `diode 511`is provided through resistor 514; however, alternating-current components are blocked from resistor 514 by choke coil y512 and are by-passed by capacitor 513. The kother stages within weighting signal generator 14 can be made in precisely the same manner as stage #5.

The'load resistors of the stages, such as 514, 614,.etc., are connected in seriesrbetween ground and output ter? minal 15 of generator 14. Thus, D.C. addition occurs for the detected signals to provide a logarithmic response above threshold.

Each of theV two stages within .thresholdicontrol-signal generator 16 in FigureZ are-constructed in precisely the same manner as stage #5 of weighting-signal-generator 14. Thus, stage #l in generator 16 is also connectedY across the cathode circuit within limiter stage #5 through blocking capacitors 516 and 517. .Hence stage #l of generator 16L is-in parallel with stage #5 of` generator 14. In a like manner stage #2 of generator 16 is in parallel with stage #6 of generator 14. The load resistors 521 and 621 in generator 16 are connected in series between ground and output terminal 20.

Hence, stages #l and #2 of generator 16 detect in the same manner as stages #5 and #6 in generator 14. However, the output of generator 14 has a different direct-current level than the combined outputs of the yfifth and sixth stages of generator 14. Furthermore the stages of generator 16 are not influenced direct-current-wise by the signal levels in other stages. Hence, generator 16 has maximum response around the threshold range of the signal.

Although this invention has been described with respect to a particular embodiment thereof, it is not to be so limited as changes and modifications may be made therein which are within the full intended scope of the. invention as defined by the appended claims.

We claim:

l. An angular-modulation diversity combiner system including at least a pair of variable-threshold receivers,.

a plurality of amplier-limiters respectively connectedV in series with said receivers, each amplier-limiter comprising a plurality of tandem-connected stages, a plurality of frequency-modulation discriminators respectively connected to the outputs Vof said amplitler-lirnitcrs, a plurality of logarithmic weighting-signal generators, each connected to stages of a respective amplier-limiter, a plurality of threshold-control signal generators also respectively connected to some stages of said amplifierlimiters, an output of each of said threshold-control generators being connected to a respective receiver to vary its threshold, a plurality of noise-cutoir circuits, each including a phase-splitter connected to an output of a respective discriminator and providing opposite phased outputs, a pluralityof cathode-followers each including a cathode and grid, with the cathode and grid of each respectively connected to the opposite-phased outputs of a respective phase splitter; a weighting-signal disconnect circuit including a direct-current amplier and having an input connected to an output of said threshold-control signal generator, an output of each disconnect circuit being connected to the grid of a respective cathodefollower; a signal-adding circuit connected between ground and the cathode of each cathode-follower, and means connecting the output of each weighting-signal generator to the grid of a respective cathode-follower.

2. A combiner as defined in claim 1 in which said signal-adding circuit includes at least one transistor as a load, means connecting a collector of said transistor to the cathode of said cathode-follower, means connecting the emitter o f said transistor to ground, and means for maintaining a iXed bias between ground and the base of said transistor.

3. A combiner system as dened in claim 1 in which said weighting-signal disconnect circuit includes at least one transistor having a base, emitter and collector, means connecting the collector of said transistor connected to theV grid of a respective cathode-follower, means connecting the emitter of said transistor to ground, and a Zener diode connected between the base and an output of said direct-current amplifier. 4

4. A combiner system as deined in claim 1, said weighting-signal generators comprising a plurality of detectors having inputs connected to outputs of respective stages of one of said amplier-limiters, said detectors being connected in series to provide the output of said generator, low-pass filtering means connected between the output of said generatorand the grid of a respective one of said cathode followers; said threshold-control signal generator comprising at least one detector stage connected to an output of one of the stages of said ampliier-limiter to respond to the threshold condition of said receiver.

5. An angular-modulation diversity-combiner system including -at least a pair of receivers receiving respectiveinput signals, with respective modulation detectors -for detecting the respective receiver outputs, a signal combiner comprisinga plurality of cathode-followers having a common load, noise-cutoff circuits connected betweef the outputs of said `detectors and `respective cathode 'followers, each of said noise-cutoff circuits including av being connected to the signal electrodes of the respectivel cathode-followers to control their currents.

6. A combiner system as deiined in claim 5 in which` said common load includes at least one transistor having base, emitter and collector, means connecting a collector of said transistor to the cathode electrode of each of said cathode-followers, resistive means connecting the emitter of said transistor to ground to bias it for commonbase operation, and means for maintaining a fixed biasy between ground and a base of said transistor.

7. A comb-incr as dened in claim 5, including a plurality of weighting-signal disconnect circuits, inputs of said disconnect circuits respectively connected to the weighting-signal outputs of said logarithmic-detectionV means, said disconnect circuits being respectively actuated in response to particular levels of said input signals, and means connecting outputs of said disconnect circuits respectively to the signal electrodes of said cathode-followers.

8. A combiner as dened in claim 7, wherein said weighting-signal disconnect circuit includes a transistor with a collector, base and emitter, the collector and emitter connected between the signal electrode of a respective cathode-follower and ground, a direct-current amplifier having an output connected to the base of said transistor, and means connecting an input of said directcurrent amplier to an output of said logarithmic-detection means.

References Cited in the le of this patent UNITED STATES PATENTS 2,364,952 Crosby Dec. 12, 1944 2,644,885 Atwood July 7, 1953 2,648,765 Young Aug. 1l, 1953 2,835,799 Day May 20, 1958 OTHER REFERENCES Proc. I.R.E., October 1955, pp. 1281-1289, Diversity Reception in UHF Long Range Communications, Mack. 

